It is known that organic polymers each containing, in the molecule thereof, at least one reactive silicon group have a property of being each crosslinked even at room temperature by siloxane-bond-formation which follows, for example, a hydrolysis reaction of the reactive silicon group by effect of moisture or some other, so as to give a rubbery cured product.
“Patent Document 1” and others disclose, among these reactive-silicon-group-containing organic polymers, polymers wherein their main chain skeletons are a polyoxyalkylene polymer, a saturated hydrocarbon polymer, and a poly(meth)acrylate copolymer, respectively. These polymers have been already industrially produced, and used widely for sealing materials, adhesives, coating materials, paints and other articles.
The polymers have been increasing in share in the market over 30 years, particularly, for use as sealing materials for architecture. In recent years, detached houses in which siding boards are used have been become a main current; and a sealing material has been used also as a joining material between the siding boards. However, the siding boards are porous boards; thus, it has come to be understood that while the boards are exposed to the outdoors over a long term, the plasticizer is being absorbed little by little into the boards so that the cured product tends to be being lowered in elongation. The plasticizer used in the sealing material for the siding boards is a phthalic acid ester, which has a low molecular weight. It is known that the tendency can be improved by using a high molecular weight plasticizer instead of the low molecular weight plasticizer. However, the advantageous effect thereof is insufficient. The so-called reactive plasticizer, which is a high molecular weight plasticizer having reactive silicon groups at its terminals or main chain, reacts with an organic polymer having a reactive silicon group. Thus, after the sealing material precursor is cured, bleeding-out is not easily caused. This technique has already been known. However, according to conventional methods for introducing the silicon groups, the proportion of molecules having terminals into which the silicon groups are introduced is at most 80 percent, so that a component into which no silicon group is introduced remains. As a result, a limitation is imposed on the bleeding-out-improving effect.
When a sealing material for architecture or an external wall is deteriorated with time, a paint may be applied onto the sealing material or wall. However, there is frequently caused a problem that a coat of the paint applied onto the sealing material comes to be stained. This is because its low molecular weight plasticizer, such as a phthalic acid ester, shifts into the paint coat and then the paint coat softens, whereby dirt adheres thereto. In this case also, the actual situation can be made better by use of a high molecular weight plasticizer or a reactive-silicon-group-introduced reactive plasticizer. However, the advantageous effect of the use is insufficient.
Even when a wall material is stretched or shrunken by effect of, for example, temperature, a sealing material thereon for architecture needs to follow the wall material. The sealing material is therefore desired to be low in modulus and high in elongation and further high in elastic restorability. In order that a cured product of a curable composition containing a reactive-silicon-group-having organic polymer can exhibit the properties of low modulus and high elongation, several methods are known. One of the methods is a method of increasing the quantity of a plasticizer therein. However, the method causes problems that the elastic restorability is declined and further the cured product is deteriorated in weather resistance. Alternatively, there is a method of decreasing, in the reactive-silicon-group-having organic polymer, the number of its silicon groups contained in each molecule of the polymer. However, tackiness of the surface of the cured product (hereinafter referred to as remaining tackiness) is unfavorably generated. There is also a method of increasing the molecular weight of the organic polymer while the number of the silicon groups contained in each molecule of the polymer is made constant. However, the viscosity of the organic polymer rises, and with the rise, the viscosity of the curable composition also rises to cause problems that the curable composition is deteriorated in workability and further lowered in elastic restorability.